The word subito in sheet music means "suddenly" and is followed by another instruction, such as fortissimo, "very loud," or pianissimo, "very soft." It means to follow that instruction instantly and suddenly.
Subito can be followed by any kind of instruction intended to create a surprising contrast for the listener. In addition to fortissimo or pianissimo, or even just forte or piano ("loud" or "soft"), you might see it followed by a variety of other instructions. For example, subito vivace means "suddenly fast," and subito diminuendo means "suddenly getting softer." Subito is often abbreviated to sub.